Now we're cooking.
Altaic, do you have any other requirements like power output, etc.? Cost, efficiency, and complexity are other tradeoffs. And finally, how much experience do you have building electronics stuff?
For example, one of the simplest amps to build is the Son of Zen, except it requires a beefy power supply. That and the heatsinks probably dominate the cost (as peranders said).
My Bwahahaha meant that it will always cost more than you think it will. You may want to start hunting up surplus shops in your area.

The nice thing about DIY is that you can build something that you can't buy in a shop. Commercial stuff is allmost allways the cheaper option. But if you want something special, you gonna have to build it yourself: enter DIY audio.

I've got a fair amount of electronics experience. Recently, I had to solder some new capacitors (and a few other little things) on my overclocked Athalon motherboard. The old ones blew up.

It's amazing how resilient "delicate" computer components are.

I first found out about Elliot's 60-100W amp (project 3A), and I thought it might be fun. But since then I've been learning about new "fancy" amp technologies... I'm now thinking of looking for a switching design, or as JoeBob suggested a Class-D or Class-T. I've heard of Class-Js, too, but I'm not sure what their deal is.

Altaic, can I just ask why you want such a high power amp? It's a common misconception that "more" is better in audio. Unless you are seriously going to raid junk markets, I think you will have a great deal of difficulty in producing a very good sounding 500W amp within that budget. Most people seriously overstate the amount of power they need, for all but the most absurdly inefficient speakers 100W is more than enough for a typical domestic situation. Of course everyone is different but to give you an example, I have a 100W amp and have never, and I do mean NEVER, got it anywhere near maximum power, indeed it would typically run at no more than a few watts (remember it's not a linear scale).

In terms of "bang for your buck" I believe that you would be far happier with the end result if you looked at amps up to the 100W mark. You should be able to produce a very good sounding amp within that budget.

Pete:
Thanks for the advice. I don't specifically need a 400W or 500W amp, but they are more interesting to build than <100W ones. Also, I can use cheap parts if I only want to use, say, 150W of the design's spec. But I like flexibility, and I may actually need high power in the not-so-distant future (not for a home stereo system).